The present invention relates to an improved circuit for identifying stereo and dual audio signals. In particular, the present invention provides a circuit for identifying stereo and dual audio signals which can be readily integrated into a package having fewer external pins.
Conventional integrated circuits (ICs) which identify stereo and dual audio signals require numerous external connection pins. Such an abundance of pins increases overall production costs.
FIG. 1 is a block diagram of a conventional circuit which identifies stereo and dual audio signals. The circuit includes an FM detector 1 which receives a second intermediate frequency (2nd IF) signal input, a bandpass filter 2, an AM detector 3 which receives the bandpass-filtered output from the FM detector, and a filtering capacitor 4. The circuit also includes an amplifier 5 having a first input receiving the capacitor-filtered output from the AM detector, and a second input receiving a signal derived from a series connected resistor 6 and a voltage source 7 between the second input and ground, and a feedback resistor 8 connected between the output of amplifier 5 and the second input. The circuit further includes a phase detector 10 which accepts as inputs, dual outputs from a voltage controlled oscillator 9, the output of amplifier 5, and a signal apparent at a common node between resistor 6 and voltage source 7, wherein voltage controlled oscillator 9 accepts as an input, the output of phase detector 10 through a low-pass filter 11.
In operation, FM detector 1 in the foregoing circuit detects the presence of an FM wave on the 2nd IF input. Bandpass filter 2 receives and filters the FM-detected signal to thereby detect a predetermined pilot signal. AM detector 3 detects the presence of an AM wave modulating the pilot signal. Capacitor 4 removes any DC component riding on the detected AM wave to prevent its transmission to amplifier 5. Amplifier 5 amplifies the detected AC component of the AM wave in order to drive the next stage of the circuit. Due to variations in the DC level of the AM wave resulting from varying degrees of modulation in the broadcasting transmission, the state of the electrical field in the broadcasting reception, etc., the output of AM detector 3 cannot be used as a direct, or uncompensated input to phase detector 10 which performs phase detection in a regular DC bias state. Therefore, the output of amplifier 5 having AC components detected using capacitor 4 is used as an input of phase detector 10. The voltage controlled oscillator 9, phase detector 10, and low-pass filter 11 function as a phase-locked loop.
When the foregoing circuit is integrated, capacitor 4 remains external to the IC and must communicate through two external pins. The external attachment of capacitor 4, thus, drives up the pin count, and the corresponding production costs for conventional ICs incorporating a stereo and dual audio signal identifying circuit.